C#(WPF版)使用PDFSharp渲染TIFF时PDF出现灰色覆盖层问题

I’ve run into similar CMYK rendering quirks with PDFSharp before, so let’s break down what’s likely going on and how to fix it:

What’s Causing the Gray Overlay? #

Your TIFF’s "undefined color" area inside the circle is probably a transparent/alpha-channeled region, and PDFSharp’s default CMYK handling fills this space with a neutral gray (like CMYK 0,0,0,10) instead of pure white. The faded border is a side effect of this gray layer blending with the original image. Since your PDF is set to CMYK, RGB white fallbacks won’t apply—we need to explicitly use CMYK pure white (0,0,0,0).

Fixes to Try #

1. Pre-Fill the PDF Page with CMYK White

Before drawing your TIFF, fill the entire page with pure CMYK white. This ensures any transparent areas in the TIFF show through to a white background instead of the default gray:

PdfDocument document = new PdfDocument();
document.Options.ColorMode = PdfColorMode.Cmyk;
PdfPage page = document.AddPage();
// ... your existing page setup code ...

XGraphics gfx = XGraphics.FromPdfPage(page);
// Fill page with CMYK pure white (0 cyan, 0 magenta, 0 yellow, 0 black)
gfx.DrawRectangle(new XSolidBrush(XColor.FromCmyk(0, 0, 0, 0)), 0, 0, page.Width, page.Height);

// Now draw your image
Assembly myAssembly = Assembly.GetExecutingAssembly();
Stream bImgStream = myAssembly.GetManifestResourceStream([path_to_tiff_file]);
XImage bImg = XImage.FromStream(bImgStream);
gfx.DrawImage(bImg, x, y, w, h);

2. Preprocess the TIFF to Replace Transparent/Placeholder Pixels

If the first fix doesn’t work, your TIFF might have an alpha channel or placeholder gray pixels that PDFSharp isn’t handling correctly. Use WPF’s imaging APIs to convert these regions to CMYK white before loading into PDFSharp:

// Load TIFF into WPF BitmapSource
BitmapImage bitmap = new BitmapImage();
using (Stream bImgStream = myAssembly.GetManifestResourceStream([path_to_tiff_file]))
{
    bitmap.BeginInit();
    bitmap.CacheOption = BitmapCacheOption.OnLoad;
    bitmap.StreamSource = bImgStream;
    bitmap.EndInit();
}

// Convert to CMYK format and make pixels editable
FormatConvertedBitmap cmykBitmap = new FormatConvertedBitmap(bitmap, PixelFormats.Cmyk32, null, 0);
WriteableBitmap editableBitmap = new WriteableBitmap(cmykBitmap);

Int32Rect rect = new Int32Rect(0, 0, editableBitmap.PixelWidth, editableBitmap.PixelHeight);
int stride = editableBitmap.PixelWidth * 4;
byte[] pixels = new byte[stride * editableBitmap.PixelHeight];
editableBitmap.CopyPixels(rect, pixels, stride, 0);

// Replace placeholder gray pixels with CMYK white (adjust condition to match your TIFF's actual data)
for (int i = 0; i < pixels.Length; i += 4)
{
    // For CMYK32, pixel order is C, M, Y, K
    if (pixels[i] == 0 && pixels[i+1] == 0 && pixels[i+2] == 0 && pixels[i+3] > 0 && pixels[i+3] < 100)
    {
        pixels[i] = 0;     // C
        pixels[i+1] = 0;   // M
        pixels[i+2] = 0;   // Y
        pixels[i+3] = 0;   // K (pure white)
    }
}

// Write back modified pixels and convert to XImage
editableBitmap.WritePixels(rect, pixels, stride, 0);
using (MemoryStream ms = new MemoryStream())
{
    TiffBitmapEncoder encoder = new TiffBitmapEncoder();
    encoder.Frames.Add(BitmapFrame.Create(editableBitmap));
    encoder.Save(ms);
    ms.Seek(0, SeekOrigin.Begin);
    
    XImage bImg = XImage.FromStream(ms);
    XGraphics gfx = XGraphics.FromPdfPage(page);
    gfx.DrawImage(bImg, x, y, w, h);
}

3. Verify Your TIFF’s Raw Color Data

Open the TIFF in an editor like Photoshop and check the CMYK channels. It’s possible the "undefined color" area is actually a light gray (non-zero K value) instead of true transparency. If that’s the case, edit the TIFF directly to set those pixels to 0,0,0,0 before rendering to PDF.

内容的提问来源于stack exchange，提问作者user3440145